The invention relates to top separator devices for digesters used to process cellulosic fiber chips into pulp fibers. The invention particularly relates to top separators having two or more sections from which may be drawn cooking liquor of different concentrations.
Digesters are pressure-cooking vessels used to process a slurry of cellulosic fibrous material, such as wood chips, and cooking liquor. The digester applies pressure and cooking liquor to process the chips into cellulose pulp fibers from which paper products may be made. A chip feed system supplies to the digester vessel a slurry of fiber chips and liquor. A top separator, generally in the top of the digester vessel, condenses the chip slurry from the chip feed system by extracting a portion of the liquor from the slurry.
A continuous digester generally includes a pressure vessel that is fully or partially filled with a slurry of the cellulosic fiber chips and cooking liquor. Wood chips, or other comminuted cellulosic fibrous material, are typically fed to the inlet of a continuous digester using the chip feed system, e.g., a wood chip slurry feed system. The feed system typically includes equipment for de-aerating, heating, steaming, pressurizing, and adding cooking liquor to the chips before transferring the slurry of chips and liquor to the digester vessel. In a hydraulic digester, the vessel is completely filled with a slurry that is under pressure and heat in the vessel. In a vapor-phase digester, the chip slurry is introduced to a pressurized gas space at an upper interior portion of a vessel. The lower portion of the vessel is filled with the chip slurry. A top separator concentrates the slurry by providing a means of removing liquid from the slurry, and distributes the slurry into the upper section of the digester vessel. From the top separator, the chips are processed in the digester vessel in a conventional manner.
The top separator removes cooking liquor from the slurry and distributes the wood chips to the digester vessel. A top separator typically includes a screw conveyor. The slurry of chips and liquor are fed to the conveyor, via a conduit that is connected to an external chip feed system. In the top separator as the chip slurry is carried by the screw conveyor, some of the liquor in the slurry is extracted out of the conveyor through a screen drum. The screen drum surrounds the screw conveyor to allow liquor to flow out of the chips being conveyed to the top of the separator. The liquor is conventionally extracted in a single stream from the screen drum of a top separator. Liquor may also be added to the slurry in the separator. The screen drum of the separator may be divided to provide a chamber to collect liquor from the slurry and another chamber(s) to introduce liquor through the screen into the slurry, as is shown in U.S. Pat. Nos. 6,332,954 and 6,361,649. However, these patents do not disclose or suggest extracting multiple streams of liquor from a top separator.
There is a need to extract multiple streams of liquor from a top separator. The multiple streams each have a different effective alkali (EA) concentration. There are requirements in the digester and chip feed system for liquor streams having various levels of EA. There is a long-felt need for a ready source of multiple streams of liquor having various EA levels. These requirements for liquor streams having multiple levels of EA can be satisfied, at least in part, by extracting multiple streams from a top separator.
In one embodiment, the invention is a top separator for a vapor-phase or hydraulic digester having a liquor extraction compartment divided into multiple chambers to allow for the extraction of multiple liquor streams each having a different EA concentration. These liquor streams extracted from the top separator are routed to various applications in the chip feed system, digester vessel or other sections of a pulp processing system.
At the screw output of the top separator, the remaining condensed slurry, i.e., the slurry with a portion of the liquor removed, containing the wood chips is fed from the top separator to other portions of the digester vessel. Additional liquor may be introduced to the vessel at or just below the top separator. This additional liquor may be drawn from one of the chambers of the top separator.
In another embodiment, the invention is a method for extracting liquor from a cellulose slurry for a continuous digester comprising the steps of: providing the cellulose slurry of liquor and chips of cellulose fiber material to a separator; extracting a first stream of liquor from the separator; extracting a second stream of liquor from the separator, where the second stream has an effective alkali (EA) concentration greater, typically at least 5 gr (as NaOH) EA/1 greater but preferably more than 10 gr (as NaOH) EA/1 greater, than the EA concentration of the first steam; outputting the first stream from the separator and outputting the second stream from the separator separately from the second stream, and outputting a condensed slurry from the separator to a digester vessel.
In a further embodiment, the invention is a method for extracting liquor from a cellulose slurry for a continuous digester comprising the steps of: providing the cellulose slurry of liquor and chips of cellulose fibrous material to a separator; extracting a first stream of liquor from the separator; extracting a second stream of liquor from the separator, where the second stream has a temperature greater, typically at least 7xc2x0 C. warmer (and preferably 15xc2x0 C. warmer), than the temperature of the first stream; outputting the first stream from the separator and outputting the second stream from the separator separately from the second stream, and outputting a condensed slurry from the separator to a digester vessel.
In another embodiment the invention is a separator for a continuous digester in a cellulose fiber processing system comprising: an inlet receiving a slurry having fiber chips and liquor; a slurry conveyor having a slurry flow path, including a first path portion near the inlet, and a second path portion near a slurry outlet to the conveyor; where the slurry outlet is downstream of the slurry flow path and provides condensed slurry to a digester vessel; a screen adjacent the conveyor and arranged along the flow path, wherein the screen is porous to allow passage of liquor and block fiber chips; a first liquor chamber adjacent the first path portion and on a side of the screen opposite to the conveyor; a second liquor chamber adjacent the second path portion and on a side of the screen opposite to the conveyor; a first liquor stream outlet coupled to the first liquor chamber, and a second liquor stream outlet coupled to the second liquor chamber.
In a further embodiment the invention is a separator for a continuous digester in a cellulose fiber processing system comprising: an inlet receiving a flow of a fiber chip and liquor slurry; a screw conveyor having a slurry flow path along a screw axis, including a first screw portion near the inlet, and a second screw portion near a slurry outlet to the conveyor; said slurry outlet being downstream of the slurry flow path and providing a condensed slurry to a digester vessel; a cylindrical screen coaxial to the conveyor and arranged along the flow path, wherein the screen is porous to allow passage of liquor and block fiber chips; a first annular liquor chamber adjacent the first screw portion and having the screen forming an inner wall to the chamber; a second liquor chamber adjacent the second screw portion and having the screen forming an inner wall to the chamber; a first liquor stream outlet coupled to the first liquor chamber, and a second liquor stream outlet coupled to the second liquor chamber.